This invention relates to heavy denier thermoplastic monofilaments, and more particularly relates to heavy denier thermoplastic monofilaments having high tenacity/high knot strength and high tensile uniformity and a process and apparatus for making such monofilaments.
U.S. Pat. Nos. 4,009,511 and 4,056,652, which are incorporated herein by reference, disclose heavy denier, polyamide monofilaments and a process for their preparation. The process includes the steps of spinning, quenching and drawing a heavy denier, polyamide monofilament in first and second draw stages to a total draw ratio of at least 5.5X. In the first draw stage, the monofilament is exposed to a steam atmosphere where it is drawn at a ratio of at least 3.5X. In the second stage, the monofilament is stretched at a ratio of at least 1.3X in a radiant heating zone. The process disclosed in U.S. Pat. Nos 4,009,511 and 4,056,652 produces a monofilament having a deoriented surface layer having an orientation less than the orientation of the core and has a refractive index, n.sup..parallel., of less than 1.567 and the core has a refractive index, n.sup..parallel., of greater than 1.57.
While the disclosed process produces monofilaments with high strength and high loop tenacities, the uniformity of tensile properties is not as high as is desired for some end uses. Furthermore, the process of U.S. Pat. Nos. 4,009,511 and 4,056,652 is not easily adapted to produce monofilaments with different deniers at high process speeds.